King
Arthur had existed for centuries in mythology poetry and legend but the first
narrative account of Arthur's reign is found in Geoffrey of Monmouth's 12th
century Latin work Historia Regum Britanniae (History of the Kings of
Britain), an entertaining and epic of British kings from the legendary Trojan
exile Brutus to the 7th century Welsh prince Cadwallader. Geoffrey places
Arthur in the same post-Roman period as the Historia Brittonum and Annales
Cambriae.

He
introduces Arthur's father, Uther Pendragon, and his magician advisor Merlin,
and the story of Arthur's conception, in which Uther, disguised as his enemy
Gorlois by Merlin's magic, fathers Arthur on Gorlois' wife Igerna at Tintagel.
On Uther's death, the fifteen-year-old Arthur succeeds him as king and fights a
series of battles, similar to those in the Historia Brittonum,
culminating in the Battle of Bath, and then defeats the Picts and Scots,
conquers Ireland, Iceland, Norway, Denmark and Gaul, and ushers in a period of
peace and prosperity which lasts until the Roman emperor Lucius Tiberius
demands tribute. Arthur refuses, and war follows.

Arthur
and his warriors, including Caius, Bedver and Walganus, defeat Lucius in Gaul,
but as he prepares to march on Rome, Arthur hears news that his nephew
Modredus, whom he had left in charge of Britain, has married his wife
Guanhumara and seized the throne. Arthur returns to Britain and defeats and
kills Modredus on the river Camblam in Cornwall, but he is mortally wounded. He
hands the crown to his kinsman Constantine and is taken to the isle of Avalon
to be healed of his wounds, never to be seen again.

Geoffrey's
Historia became very popular and influential and was translated into Norman
French verse by Wace, who introduced the Round Table, and Middle English verse
by Layamon. It fed back into Welsh tradition, with three different Welsh prose
translations appearing, and material in the Welsh triads deriving from it.